1. Field of the Invention
The present invention relates to a process for producing a ceramic composite suitable for materials in various uses requiring strength and toughness such as a part of an internal combustion engine for automobiles such as a glow plug, a turbine wheel of a turbo charger, a piston cap, a lining of a cylinder, a tappet, an exhaust valve, a hot plug in a sub-combustion chamber of a diesel engine, a part of an aircraft or a spacecraft such as a fan, an air compressor of a jet engine, a housing of a combustion chamber, a heat insulator of a nose cone, a slidable part of a pump, a die, a tool, cutlery, a thread guide, heat resisting pipe, a pipe for a high-temperature gas, a crucible for dissolution, a roll for a rolling mill, etc.
2. Description of the Prior Art
Various processes which are not accompanied with reaction during sintering are known as processes for producing ceramic composites. JP-A-SHO 58-120571 discloses a process for producing TiB.sub.2 -ZrO.sub.2, TiC-ZrO.sub.2, TiN-ZrO.sub.2 and TiCN-ZrO.sub.2 composites containing TiB.sub.2, TiC, TiN and TiCN as respective main components. JP-A-SHO 47-35011 (corresponding to U.S. Pat. Nos. 3,808,012 and 3,859,399) discloses a process for producing a ceramic composite containing TiB.sub.2, B.sub.4 C, SiC and Si. JP-A-SHO 49-85115 discloses a process for producing a ceramic composite containing TiB.sub.2 and WC-system cemented carbide. JP-A-SHO 58-217463 and JP-A-SHO 62-292678 disclose processes for producing a ceramic composite containing TiB.sub.2 and Al.sub.2 O.sub.3. JP-A-SHO 59-118828 (corresponding to U.S. Pat. No. 4,514,355) discloses a process for producing a ceramic composite containing TiB.sub.2, BN and Al.sub.2 O.sub.3. JP-A-SHO 60-226459 discloses a process for producing a ceramic composite containing TiB.sub.2, B.sub.4 C and TiN. JP-A-SHO 61-270265 discloses a process for producing a ceramic composite containing TiB.sub.2, TiC and SiC.
As processes for producing ceramic composites containing ZrC, the process disclosed in "Journal of Material Science" (vol. 22, page 1135-1140, 1987) wherein ZrC is combined with Al.sub.2 O.sub.3 and the process disclosed in "Poroshkovaya Metallurgiya" (vol. 149, page 61-64, 1975) wherein ZrC is combined with ZrB.sub.2 are known.
It is common to the conventional processes described above that raw powders (hereinafter also called "starting materials") comprising mixed powders consisting of the same compositions as those of finally obtained ceramic composite are prepared, a desired article is formed with the raw powders and the obtained green article is sintered. In such conventional processes, however, the following problems are present.
First, there is a problem in obtaining obtain a sintered material dense and having fine crystal grains. Because, the particle size of sintered materials generally becomes greater than that of raw powders on account of the growth of crystal grains, the particle size of the raw powders should be as small as possible in order to obtain a sintered material which is dense and has fine crystal grains. In practice, however, it is difficult to obtain raw materials of fine ceramic powders with a commercial base, and particularly non-oxide ceramic powders comprising fine powders having particles of less than a submicrometer size are not available under present circumstances. Accordingly, the particle size of the raw materials which can be industrially applied is limited to an extent, and the densification and fining of obtained sintered materials are also limited. If an obtained sintered material is not dense and if the crystal grains thereof are not sufficiently fine, high mechanical properties thereof cannot be attained.
Moreover, although raw materials of ceramic powders should desirably be as fine as possible, as described above, the raw powders are liable to aggregate and become difficult to disperse as they become finer. Since aggregation of raw powders occurs more or less in the conventional processes, such aggregation causes the segregation of the mixed raw powders, whereby the structure of the obtained sintered material becomes nonuniform and the mechanical properties thereof decreases.
Further, in the conventional processes, fairly high temperature, high pressure and a long processing time are required as sintering conditions in order to make pores as small as possible and obtain a denser sintered material. Accordingly, applicable sintering conditions are limited by the kind, particle size, purity etc. of raw powders, and the sintering conditions are inevitably fairly high temperature, high pressure and long duration. Satisfactory sintering cannot be achieved under the conditions of relatively low temperature, low pressure and short duration in the conventional processes.
Furthermore, particularly in the conventional process wherein metal powders are used as a starting material, there is the following problem. If a part of the metal powders remain in the sintered material without the reaction and without converting to a ceramic (i.e. as they are), not only the sintered material becomes porous because the metal powders remain with segregation, but also the mechanical properties of the sintered material greatly decrease because the segregated portions become origins for the fracture of the material.
Furthermore, as to processes accompanied by reaction during sintering, reaction-sintering process and self-combustion process are known.
The reaction-sintering process is a process wherein, after forming a desired article with raw metal powders by using one of various forming methods, a ceramic such as Si.sub.3 N.sub.4 and SiC is produced in the article by a special heat treatment. For example, after forming an article with Si powders, the article is nitrified in ammonia or a N.sub.2 atmosphere, and Si.sub.3 N.sub.4 is produced by the following reaction 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 which occurs in the nitriding step.
The self-combustion process is a process wherein mixed powders including metal powders which should become a component of a target inorganic compound having a high melting point are ignited, and self propagation of the reaction is performed by utilizing the heat of the reaction, thereby achieving a high composition rate. For example, a mixture of Ti powders and B powders is ignited and the reaction of Ti+2B.fwdarw.TiB.sub.2 is performed. This process can be applied not only to the composition of a ceramic consisting of a single compound such as TiB.sub.2 or TiC but also to the composition of a ceramic composite such as TiB.sub.2 -TiC system.
In the reaction-sintering process and the self-combustion process, Si.sub.3 N.sub.4, SiC, TiB.sub.2, TiC etc. are produced by a specified solid solution process, so-called "interstitial solid solution", wherein the atoms of N, B or C are intruded into crystalline lattices of Si or Ti which is a metal. Because a large thermal energy is generated by this intrusion, the condition of the sintering, in general, is limited to a specified condition of a fairly high temperature. Accordingly, it is difficult to conduct a desired sintering and obtain a dense sintered material under the condition of a relatively low temperature.